It is known that such stoppers are generally provided with a set of rated overpressure and underpressure valves which serve, when the stopper is mounted on a radiator, to maintain the pressure inside the radiator between specified maximum and minimum values. The maximum pressure, for which the stopper is rated is higher than atmospheric pressure. On some occasions when the stopper is removed, this can lead to a jet of steam escaping through the resulting orifice, and hence to a risk of scalding.
This drawback is avoidable when the radiator stopper is of the screw on type for fixing over the end of an inlet tube to the radiator. This is because unscrewing the stopper over a complete turn is a progressive operation in which a passage gradually opens up in between the inside surface of the stopper and the outside surface of the inlet tube, whereby the excess steam pressure inside the radiator is released before the stopper is completely unscrewed. Also, channels can be formed in the skirt or threaded rim of the stopper to direct or guide the escaping steam in a particular direction, generally downwards, to further reduce the risk of scalding.
In known bayonet-fixing type stoppers which provide for steam to escape progressively while the stopper is being undone, the valves in the stopper bear against a seating on the water box from which the inlet tube projects. Manufacturing tolerances for water boxes, and the deformation to which they are subject in operation, make it impossible to rate the valves to accurate and reproducible settings, which can lead to the radiator functioning badly. Further, during stopper opening the steam escapes progressively via the overpressure valve and thus depends on the state thereof.
Preferred embodiments of the invention provide a bayonet-fixing stopper, in particular for the radiator of an internal combustion engine, which avoids the above drawbacks.
Further, in said preferred embodiments, the structure is simple and easy to assemble, and the cost is much the same as prior art bayonet-fixing stoppers.